Heterogeneity of the hippocampus: effects of subfield lesions on locomotion elicited by dopaminergic agonists

Dissociable regulation of instrumental action within mouse prefrontal cortex

Evaluation of the behavioral 'costs', such as effort expenditure relative to the benefits of obtaining reward, is a major determinant of goal-directed action. Neuroimaging evidence suggests that the human medial orbitofrontal cortex (mOFC) is involved in this calculation and thereby guides goal-directed and choice behavior, but this region's functional significance in rodents is unknown despite extensive work characterizing the role of the lateral OFC in cue-related response inhibition processes. We first tested mice with mOFC lesions in an instrumental reversal task lacking discrete cues signaling reinforcement; here, animals were required to shift responding based on the location of the reinforced aperture within the chamber. Mice with mOFC lesions acquired the reversal but failed to inhibit responding on the previously reinforced aperture, while mice with prelimbic prefrontal cortex lesions were unaffected. When tested on a progressive ratio schedule of reinforcement, mice with prelimbic cortical lesions were unable to maintain responding, resulting in declining response levels. Mice with mOFC lesions, by contrast, escalated responding. Neither lesion affected sensitivity to satiety-specific outcome devaluation or non-reinforcement (i.e. extinction), and neither had effects when placed after animals were trained on a progressive ratio response schedule. Lesions of the ventral hippocampus, which projects to the mOFC, resulted in similar response patterns, while lateral OFC and dorsal hippocampus lesions resulted in response acquisition, though not inhibition, deficits in an instrumental reversal. Our findings thus selectively implicate the rodent mOFC in braking reinforced goal-directed action when reinforcement requires the acquisition of novel response contingencies.

Combined lesions of GABAergic and cholinergic septal neurons increase locomotor activity and potentiate the locomotor response to amphetamine

Potentiated locomotor response to amphetamine has been associated with an increased sensitivity of the dopaminergic system and used as a model of the positive symptoms of schizophrenia in rodents. The hippocampus, through the subiculum, modulates dopamine transmission and hippocampal or subicular lesions potentiate the locomotor response to amphetamine. However, little is known about the upstream structures controlling hippocampal/subicular activity towards the regulation of dopamine transmission. The main modulatory input to the hippocampus is the septal area, composed of the medial septum and vertical limb of the diagonal band of Broca (MS/vDBB). The so-called septohippocampal pathway includes cholinergic and GABAergic fibers reaching the hippocampus through the fimbria-fornix. While electrolytic lesions of the MS/vDBB potentiate the locomotor response to amphetamine, cholinergic damage in the MS/vDBB does not affect this response. Moreover, the role of the GABAergic connections has never been investigated. Therefore, we performed in rats lesions of cholinergic or/and GABAergic septal neurons and assessed locomotor activity, (i) in an unfamiliar environment, (ii) under baseline conditions (separating light-on and light-off periods) and (iii) in response to an amphetamine challenge. While single lesions had no effects, rats with combined lesions were hyperactive in all three conditions. Thus, damage to cholinergic and GABAergic septohippocampal neurons induced locomotor alterations qualitatively comparable to those produced by hippocampal and/or subicular lesions. Our results further suggest that the septum, through both cholinergic and GABAergic fibers, modulates the functional contribution of the hippocampus/subiculum in the regulation of mesolimbic dopamine transmission.

Local hippocampal methamphetamine-induced reinforcement

Drug abuse and addiction are major problems in the United States. In particular methamphetamine (METH) use has increased dramatically. A greater understanding of how METH acts on the brain to induce addiction may lead to better therapeutic targets for this problem. The hippocampus is recognized as an important structure in learning and memory, but is not typically associated with drug reinforcement or reward processes. Here, the focus is on the hippocampus which has been largely ignored in the addiction literature as compared to the nucleus accumbens (NAc), ventral tegmental area (VTA), and prefrontal cortex (PFC). The results show that METH administered unilaterally via a microdialysis probe to rats’ right dorsal hippocampus will induce drug-seeking (place preference) and drug-taking (lever-pressing) behavior. Furthermore, both of these responses are dependent on local dopamine (DA) receptor activation, as they are impaired by a selective D₁/D₅ receptor antagonist. The results suggest that the hippocampus is part of the brain's reward circuit that underlies addiction.

Acute hippocampal brain-derived neurotrophic factor restores motivational and forced swim performance after corticosterone

BACKGROUND

Alterations in cellular survival and plasticity are implicated in the neurobiology of depression, based primarily on the characterization of antidepressant efficacy in naïve rodents rather than on models that capture the debilitating and protracted feelings of anhedonia and loss of motivation that are core features of depression.

METHODS

In adult male mice, we evaluated persistent effects of oral corticosterone (CORT) exposure on anhedonic-like behavior, immobility in the forced swim test (FST), motivational performance in the progressive ratio task, and later endogenous CORT secretion. After verifying long-term decreases in hippocampal brain-derived neurotrophic factor (BDNF) and cAMP Response Element Binding protein phosphorylation (pCREB), the ability of direct hippocampal BDNF microinfusion after CORT exposure to reverse deficits was investigated.

RESULTS

Prior CORT exposure decreased sucrose consumption, appetitive responding, and FST mobility without long-term effects on water/quinine discrimination and endogenous CORT secretion. Critically, BDNF replacement mimicked chronic antidepressant treatment (ADT) by reversing CORT-induced reductions in instrumental performance and FST mobility.

CONCLUSIONS

Together these findings link persistent alterations in hippocampal BDNF expression and CREB transcriptional activity with a persistent depressive-like state-as opposed to ADT efficacy. These results identify hippocampal BDNF as an essential molecular substrate that bidirectionally regulates appetitive instrumental behavior. Additionally, we suggest this CORT model might provide a powerful tool for future investigation into the neurobiology of complex stress-associated depressive symptoms that persist long after stress exposure itself.

Modulation by the dorsal, but not the ventral, hippocampus of the expression of behavioural sensitization to amphetamine

Although the dorsal hippocampus (DH) and the ventral hippocampus (VH) densely innervate the nucleus accumbens, which mediates the expression of behavioural sensitization, the respective and specific contribution of DH and VH in the expression of behavioural sensitization to amphetamine has not been investigated. In the present study, we investigated how lidocaine infused in DH or VH modulated behavioural locomotor sensitization induced by repeated administration of systemic amphetamine. Rats, well habituated to their environmental conditions and experimental protocol, were given repeated administration of systemic amphetamine. Once behavioural sensitization was developed, rats were challenged with amphetamine and infused with saline (controls) or lidocaine into DH or VH. We found that reversible inhibition by lidocaine of DH, but not VH, blocks the expression of behavioural sensitization to amphetamine. Control animals injected with saline solution do express behavioural sensitization. Our results bring new insights on the role of the hippocampus complex in the expression of behavioural sensitization, indicating that, in individuals well habituated to the drug-associated context, DH but not VH would play a key role. The results provide experimental evidence for clinical studies in human addicts that have demonstrated that exposure to environmental stimuli associated with drug-taking behaviour elicits craving and can promote relapse, and further suggest that in drug abusers, once addiction has occurred, the contextual and spatial conditions that are associated with drug consumption may play a critical role in the maintenance of drug abuse.

Combined damage to entorhinal cortex and cholinergic basal forebrain neurons, two early neurodegenerative features accompanying Alzheimer's disease: effects on locomotor activity and memory functions in rats

In Alzheimer's disease (AD), cognitive decline is linked to cholinergic dysfunctions in the basal forebrain (BF), although the earliest neuronal damage is described in the entorhinal cortex (EC). In rats, selective cholinergic BF lesions or fiber-sparing EC lesions may induce memory deficits, but most often of weak magnitude. This study investigated, in adult rats, the effects on activity and memory of both lesions, alone or in combination, using 192 IgG-saporin (OX7-saporin as a control) and L-N-methyl-D-aspartate to destroy BF and EC neurons, respectively. Rats were tested for locomotor activity in their home cage and for working- and/or reference-memory in various tasks (water maze, Hebb-Williams maze, radial maze). Only rats with combined lesions showed diurnal and nocturnal hyperactivity. EC lesions impaired working memory and induced anterograde memory deficits in almost all tasks. Lesions of BF cholinergic neurons induced more limited deficits: reference memory was impaired in the probe trial of the water-maze task and in the radial maze. When both lesions were combined, performance never improved in the water maze and the number of errors in the Hebb-Williams and the radial mazes was always larger than in any other group. These results (i) indicate synergistic implications of BF and EC in memory function, (ii) suggest that combined BF cholinergic and fiber-sparing EC lesions may model aspects of anterograde memory deficits and restlessness as seen in AD, (iii) challenge the cholinergic hypothesis of cognitive dysfunctions in AD, and (iv) contribute to open theoretical views on AD-related memory dysfunctions going beyond the latter hypothesis.

Amphetamine-induced hyperlocomotion in rats: Hippocampal modulation of the nucleus accumbens

Using lesions and infusions, the present study investigated the way in which and the extent to which the ventral hippocampus (vHIP) modulates amphetamine-induced hyperactivity in rats. Rats were lesioned (excitotoxic or sham) in the vHIP or were implanted with cannulae for subsequent infusions. A high dose (12.5 microg/microl) of N-methyl-D-aspartate (NMDA) was used to make excitotoxic lesions and a low dose (0.5 microg/microl) of NMDA to cause activation of the hippocampus. Lidocaine was used to inactivate the hippocampus. Lidocaine or a low dose of NMDA was infused into the vHIP in combination with either systemic injection or intra-accumbens infusions of amphetamine. The effects of these treatments on locomotor activity were measured by distance traveled in 10-min intervals for 40-60 min. Lesions and deactivation of the hippocampus attenuated amphetamine-induced hyperlocomotion, compared to the controls. Stimulation of the hippocampus augmented amphetamine-induced hyperlocomotion. The present findings provide evidence that the hippocampus exerts excitatory modulation on the expression of behavioral excitation produced by amphetamine, likely via the nucleus accumbens.

The hippocampus and appetitive Pavlovian conditioning: effects of excitotoxic hippocampal lesions on conditioned locomotor activity and autoshaping

The hippocampus (HPC) is known to be critically involved in the formation of associations between contextual/spatial stimuli and behaviorally significant events, playing a pivotal role in learning and memory. However, increasing evidence indicates that the HPC is also essential for more basic motivational processes. The amygdala, by contrast, is important for learning about the motivational significance of discrete cues. This study investigated the effects of excitotoxic lesions of the rat HPC and the basolateral amygdala (BLA) on the acquisition of a number of appetitive behaviors known to be dependent on the formation of Pavlovian associations between a reward (food) and discrete stimuli or contexts: (1) conditioned/anticipatory locomotor activity to food delivered in a specific context and (2) autoshaping, where rats learn to show conditioned discriminated approach to a discrete visual CS+. While BLA lesions had minimal effects on conditioned locomotor activity, hippocampal lesions facilitated the development of both conditioned activity to food and autoshaping behavior, suggesting that hippocampal lesions may have increased the incentive motivational properties of food and associated conditioned stimuli, consistent with the hypothesis that the HPC is involved in inhibitory processes in appetitive conditioning.

Enhancement of central dopaminergic activity in the kainate model of temporal lobe epilepsy: implication for the mechanism of epileptic psychosis

There is an increased incidence of schizophrenia-like psychosis in temporal lobe epilepsy (TLE), and several risk factors have been implicated, including the duration of epilepsy and temporal lobe neuropathology. To investigate the biological mechanism of epileptic psychosis, we examined alterations of central dopaminergic systems in the kainate model of TLE. In adult rats, kainate was microinjected into the left amygdala to induce status epilepticus. An indirect dopamine agonist methamphetamine (MAP, 2 mg/kg, i.p.) was administered before and 1 month after the kainate treatment. MAP-induced locomotor activity was significantly enhanced in the kainate group compared with the baseline (pre-kainate) level, which was antagonized by pretreatment with haloperidol. The enhancement of locomotor activity in the kainate group was significantly correlated with the density of hippocampal CA1 neurons. Although the basal extracellular dopamine concentration was significantly lower in the striatum in the kainate group than in the control group (5.5 vs 39.2 fmol/20-min sample), the maximal concentration following MAP administration did not differ between the two groups. These results clearly demonstrate that hypersensitivity of the dopamine systems develops in the chronic phase of the kainate-induced TLE model, which may be responsible for the mechanism of epileptic psychosis.

Locomotor sensitization to cocaine in rats with olfactory bulbectomy

Olfactory bulbectomy in rats has been suggested as a comprehensive animal model of affective disorders associated with an array of behavioral changes, responsivity to chronic antidepressant treatment, and alterations in limbic structures thought to be critical in the pathophysiology of affective disorders. Recent work showing increased motivational responsivity to amphetamine suggests that olfactory bulbectomy could also be a useful animal model of dual diagnosis disorders. To further investigate this possibility, we studied locomotor activity in olfactory bulbectomized rats 14 days postsurgery in response to novelty and upon acute and repeated injections of cocaine (15/mg/kg) or saline. Consistent with prior studies, lesioned animals showed greater locomotor activity in response to a novel environment and significantly heightened locomotor activation upon initial cocaine exposure. Over 7 days of repeated cocaine injections, lesioned animals also showed a presensitized pattern of activity, with a loss of incremental increases in locomotion observed in control animals. Daily saline injections produced no group differences in pre- or postinjection activity, while cocaine-treated bulbectomized rats demonstrated a decline in their daily preinjection activity. These results suggest that neural alterations caused by olfactory bulbectomy produce altered behavioral response patterns to repeated doses of cocaine, and support the study of olfactory bulbectomy as a useful neurobehavioral model for understanding substance use disorder comorbidity in mental illness.

Loss of cortical acetylcholine enhances amphetamine-induced locomotor activity

Cholinergic disturbances have been implicated in schizophrenia. In a recent study we found that intracerebroventricular (i.c.v.) delivery of the immunotoxin 192 IgG-saporin, that effectively destroys cholinergic projections from the basal forebrain to hippocampus and cortex cerebri, leads to a marked facilitation of amphetamine-induced locomotor activity in adult rats. The aim of the present experiments was to evaluate the contribution of the septohippocampal versus the basalocortical cholinergic projections for the amphetamine hyper-response seen previously in i.c.v. 192 IgG-saporin injected rats. Since i.c.v. delivery of 192 IgG-saporin also destroys a population of Purkinje neurons in cerebellum, this cell loss needs to be taken into consideration as well. Cortex cerebri and hippocampus were selectively cholinergically denervated by intraparenchymal injections of 192 IgG-saporin into nucleus basalis magnocellularis and the medial septum/diagonal band of Broca, respectively. Selective loss of Purkinje cells in cerebellum was achieved by i.c.v. delivery of OX7 saporin. Possible effects of these three lesions on spontaneous and amphetamine-induced locomotor activity were assessed in locomotor activity cages. We find that selective cholinergic denervation of cortex cerebri, but not denervation of hippocampus or damage to cerebellum can elicit dopaminergic hyper-reactivity similar to that seen in previous i.c.v. 192 IgG-saporin experiments. Our data are compatible with the hypothesis that disturbances of cholinergic neurotransmission in cortex cerebri may be causally involved in forms of schizophrenia.

Facilitation of dopamine-mediated locomotor activity in adult rats following cholinergic denervation

The dopamine hypothesis of schizophrenia postulates hyperactivity of dopaminergic neurotransmission in the mesolimbic system. However, the possible underlying causes for this dopaminergic overfunction are not well understood. Therefore, the main aim of this study was to examine the effect of central cholinergic denervation on dopamine-mediated functions. We also examined the effect of neonatal cholinergic denervation upon adult brain function. The immunotoxin 192 IgG-saporin causes severe lesions of the basal forebrain cholinergic system when infused into the lateral ventricles by targeting neurons expressing the p75 neurotrophin receptor. The toxin may also damage p75-expressing Purkinje neurons in the cerebellum. We have compared the behavioral effects of intracerebroventricular injections of 192 IgG-saporin to adult rats with that of injections to neonate rats. As expected, adult treated rats displayed an almost complete cholinergic denervation of forebrain corticohippocampal areas concomitant with a marked impairment in the Morris water maze. When tested as adults, neonatally treated animals had a less complete cholinergic denervation and showed lesser impairments in water maze behaviors. Interestingly, adult treated rats showed increased spontaneous horizontal activity and a remarkable increase in locomotor response to d-amphetamine as evidenced by increased horizontal and vertical activity. There were no marked changes of spontaneous or drug-induced locomotor activity in adult rats treated with 192 IgG-saporin as neonates. These results suggest that cholinergic denervation of the forebrain causes a marked enhancement of behavioral responses related to dopaminergic activity, probably mainly mediated presynaptically. However, it cannot be fully excluded that damage to noncholinergic systems, e.g., Purkinje cells, might contribute to the effects. The striking overreaction to dopaminergic stimuli, presumably caused by the cholinergic deficit, is discussed in relation to the suggested role of cholinergic malfunctioning in schizophrenia.

The entorhinal cortex-nucleus accumbens pathway and latent inhibition: a behavioral and neurochemical study in rats

Latent inhibition (LI) refers to the decrease in conditioned response produced by the repeated nonrein-forced preexposure to the to-be-conditioned stimulus. Experiment I investigated the effects of electrolytic lesions of the entorhinal cortex on LI in a conditioned emotional response procedure. Entorhinal cortex lesions attenuated LI. Experiments 2 and 3 investigated whether this attenuation of LI could result from a modification in nucleus accumbens (NAcc) dopamine (DA) release. Rats with entorhinal cortex lesions displayed normal spontaneous and amphetamine-induced locomotor activity, as well as normal basal and amphetamine-induced release of DA within the NAcc (assessed by microdialysis). Taken together, these results show that entorhinal cortex lesions disrupt LI in a way that is unlikely to be due to an alteration of DA release within the NAcc.

A neurobiological basis for substance abuse comorbidity in schizophrenia

It is commonly held that substance use comorbidity in schizophrenia represents self-medication, an attempt by patients to alleviate adverse positive and negative symptoms, cognitive impairment, or medication side effects. However, recent advances suggest that increased vulnerability to addictive behavior may reflect the impact of the neuropathology of schizophrenia on the neural circuitry mediating drug reward and reinforcement. We hypothesize that abnormalities in the hippocampal formation and frontal cortex facilitate the positive reinforcing effects of drug reward and reduce inhibitory control over drug-seeking behavior. In this model, disturbances in drug reward are mediated, in part, by dysregulated neural integration of dopamine and glutamate signaling in the nucleus accumbens resulting form frontal cortical and hippocampal dysfunction. Altered integration of these signals would produce neural and motivational changes similar to long-term substance abuse but without the necessity of prior drug exposure. Thus, schizophrenic patients may have a predilection for addictive behavior as a primary disease symptom in parallel to, and in many, cases independent from, their other symptoms.

The time course of the hyperactivity that follows lesions or temporary inactivation of the fimbria-fornix

Lesions of the hippocampus or the fimbria-fornix produce a pronounced hyperactivity in rats. This effect is thought to be due to the loss of glutamatergic hippocampal inputs to the nucleus accumbens, although the mechanisms involved remain unclear. It has been suggested that the hyperactivity is due to changes in accumbens dopamine receptors, possibly involving the gradual development of denervation supersensitivity. Consistent with this possibility, the present study found that fimbria-fornix transection produced hyperactivity which, although undetectable immediately after surgery, gradually became apparent and then continued to increase over the course of several days. This does not, however, preclude the possibility that there is an immediate increase in activity which is masked by the after effects of surgery. To address this issue, local anaesthetic was infused into the fimbria-fornix via chronic indwelling cannulae, in order to silence the hippocampal inputs to the nucleus accumbens. This procedure impaired spatial working memory on the elevated T-maze and resulted in immediate hyperactivity, suggesting that there may be at least two components to fornix lesion-induced hyperactivity, and that the immediate effects of mechanical fornix lesions on activity levels may be masked by the after effects of surgery per se.